1. Field of the Invention
The present invention relates generally to a mixer used for a direct conversion receiver, and more particularly, but without limitation, to a mixer for reducing a second-order intermodulation (IM2) component included in a low frequency data signal.
2. Description of the Related Art
Low frequency baseband data signals are up-converted into high frequency radio frequency (RF) band data signals in transmitters of communication systems. High frequency RF band data signals are down-converted into low frequency baseband data signals in communication system receivers. A heterodyne method or a direct conversion method may be used to transmit and receive data signals. The heterodyne method uses an intermediate frequency (IF) band, whereas the direct conversion method directly up-converts low frequency baseband data signals into high frequency RF band data signals and then directly down-converts high frequency RF band data signals into low frequency baseband data signals.
FIG. 1 is a partial circuit diagram of a conventional direct down-conversion transceiver. Referring to FIG. 1, the conventional direct down-conversion transceiver includes an antenna 101, a duplexer 102, a transmission end Tx 103, a reception end Rx 104, a low noise amplifier (LNA) 105, a surface acoustic wave (SAW) filter 106, and a mixer 110.
When the conventional direct down-conversion transceiver serves as a transmitter, a transmission signal is transmitted through the transmission end Tx 103, the duplexer 102, and the antenna 101. When the conventional direct down-conversion transceiver serves as a receiver, a signal received through the antenna 101 is input into the LNA 105 through the duplexer 102 and the reception end Rx 104. The signal amplified by the LNA 105 is input into the mixer 110 through the SAW filter 106. The mixer 110 mixes an input high frequency data signal radio frequency (RF) and a local oscillation (LO) signal and generates a low frequency data signal Vpn. In more detail, the mixer 110 generates the low frequency data signal Vpn having a frequency corresponding to a difference between frequencies of the high frequency data signal RF and the LO signal.
FIG. 2A is a graph illustrating heterodyne down-conversion, and FIG. 2B is a graph illustrating direct down-conversion. Referring to FIGS. 2A and 2B, a horizontal axis indicates a frequency, and a vertical axis indicates signal amplitude. RFx denotes a transmission signal, RFy denotes a signal adjacent to RFx, and LO denotes an LO signal. Intermodulation is a type of signal interference that is the result of two or more signals of different frequencies being mixed together. Accordingly, the adjacent signal RFy may cause noise to occur in the transmission signal RFx.
Referring to FIG. 2A, both signals RFy and RFx are down-converted into IF band signals according to the heterodyne down-conversion. In more detail, an IF band includes a frequency f1-f3 of a mixing signal of the signal RFx and the LO signal LO, and a frequency f2-f3 of a mixing signal of the adjacent signal RFy and the LO signal LO. Meanwhile, a frequency f1-f2 of a second-order intermodulation (IM2) component caused by the intermodulation between signals RFy and RFx does not belong to the IF band, and therefore the IM2 component can be easily removed by performing band reject filtering.
Referring to FIG. 2B, both signals RFy and RFx are down-converted into baseband signals according to the direct down-conversion. In more detail, a baseband includes a frequency f1-f3 of a mixing signal of the signal RFx and the LO signal LO, and a frequency f2-f3 of a mixing signal of the adjacent signal RFy and the LO signal LO. However, a frequency f1-f2 of the IM2 component caused by the intermodulation between signals RFy and RFx belongs to the baseband too, making it impossible to remove the IM2 component by performing band reject filtering. Therefore, a method of removing the IM2 component is additionally needed.
FIG. 3 is a circuit diagram of a conventional mixer 110 including a circuit used to remove the IM2 component. Referring to FIG. 3, the mixer 110 is used as one of the constituents of the conventional direct down-conversion receiver illustrated in FIG. 1.
In order to remove the IM2 component, the mixer 110 includes resistive circuits Rp, Rpc, Rn, and Rnc that are connected between a power voltage source VDD and output terminals TER1 and TER2, and a mixing unit (MU) that mixes high frequency data signals RFp and RFn and LO signals LOp and LOn and outputs low frequency data signals Vp and Vn. Iim2_d denotes a differential current of the IM2 component included in the low frequency data signals Vp and Vn. Iim2_c denotes a common current of the IM2 component included in the low frequency data signals Vp and Vn.
If a value of combination resistance of the resistors Rp and Rpc is (R+ΔR), and a value of combination resistance of the resistors Rn and Rnc is (R−ΔR), the IM2 component [Vpn(fim2)] included in the low frequency data signals Vp and Vn can be approximated according to Equation 1,Vpn(fim2)=R*Iim2—d±ΔR*Iim2—c  (1)wherein, fim2 denotes a frequency of the IM2 component.
Referring to Equation 1, proper control of a value ΔR can minimize the IM2 component [Vpn(fim2)] included in the low frequency data signals Vp and Vn. The value of ΔR can be controlled by changing the value of combination resistance of the resistors Rp and Rpc by varying a value of the resistor Rpc connected in parallel to the resistor Rp, or by changing the value of combination resistance of the resistors Rn and Rnc by varying a value of the resistor Rnc connected in parallel to the resistor Rn.
For example, if the resistor Rp is 1 kΩ, the value of the resistor Rpc must be 99 kΩ in order that the value of combination resistance of the resistors Rp and Rpc is 0.99 kΩ. In other words, it is necessary to specify a resistor Rpc with a value more than a hundred times larger than the resistor Rp in parallel in order to reduce the combination resistance value by 1%. However, the resistor having such a large resistance value needs a large area in order to be realized in a system chip, which increases the overall area of the system chip. Therefore, an additional method is needed for high integration of the system chip.